In the oil and gas industry acoustic tools are used to provide operationally significant information about seismic events occurring during production of a well. Borehole seismic data can be utilized to determine subsurface stratigraphy and refine surface seismic data. Micro-seismic data may be generated by formation fracturing—induced or natural. Worthwhile seismic data can be gathered on a continuing or recurrent basis to monitor subsurface formations and reservoirs during production operations of a well. In this, the gathering of data on a continuous basis from a downhole location assists in optimizing extraction of gas or oil deposits from a producing well.
In the past, downhole seismic data has been acquired by a number of different techniques. Sensors have been mechanically coupled outside of casing and cemented in place. In other instances, sensors have been coupled to production tubing, inside a well casing, or in association with production packers. In devices of the internal kind deployment is an issue and maintaining an operative sensor that does not interfere with normal production activity can be a challenge.
Another seismic system that has been used is a wireline tool which is connected to a signal wire from a surface location and is lowered into a cased hole to measure/monitor seismic and micro-seismic events. A wireline tool is normally used to monitor signals for a short period of time such as a few days or a few weeks.
Another seismic monitoring system involves drilling a monitoring well that is substantially parallel with a production well and seismic or micro-seismic events can be monitored from this separate monitoring well. In this environment a seismic tool can be a permanent device which is deployed on tubing and then run into a deep cased hole to monitor subsurface formations and reservoirs during the production life of the adjacent well. A permanent seismic device is normally used to monitor signals for a longer period of time such as a few weeks to ten years or more. The gathering of seismic data on a continuous basis assists in optimizing extraction of gas or oil deposits.
In the past, permanent seismic tools have been lowered into monitoring wells drilled adjacent to a production well on tubing that carries the seismic tool. In this system the seismic tool was biased against a surrounding casing by one or more bow springs. The weight of tubing over long distances, however, such as four thousand meters or more, in a deviated or even slightly sloping hole can counteract the normal operative force provided by a bow spring arrangement and compromise solid coupling contact between a seismic sensor and the wall of a surrounding casing.
In at least one other previously known design a seismic tool has been designed to be mounted on the periphery of a mandrel, or short length of tubing, and lowered into a borehole casing. At a desired depth the seismic tool is actuated to extend laterally and into solid contact with a casing side wall. Some of these tools are designed to provide azimuthal orientation before being locked in place. Upon further activation the tool is designed to be released from contact with the casing wall so that the seismic tool can be retrieved. Seismic tools of this design, although theoretically appealing, are somewhat complicated in design detail and are subject to failure during operation over long periods of time, particularly during release operations. Moreover, tools of this design are more expensive than previously known bow spring based designs.
Although permanent or semi-permanent seismic sensors have been known in the past, a need exists for an enhanced reliable, rugged and cost effective way of placing and maintaining a seismic tool in a downhole monitoring location for recording seismic and micro-seismic events in a nearby production well geologic zone throughout the operating life of the production well.